Before you go to another country it is a great help if you know the language and some of the customs of the country.
    When people meet each other for the first time in Britain, they say “How do you do?” and shake hands. Usually they do not shake hands when they just meet or say goodbye. But they shake hands after they haven't met for a long time or when they will be away from each other for a long time.
    Last year a group of German students went to England for a holiday. Their teacher told them that the English people hardly shake hands. So when they met their English friends at the station, they kept their hands behind their backs. The English students had learned that the Germans shake hands as often as possible, so they put their hands in front and got ready to shake hands with them. It made both of them laugh.

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2010年2月，我国科学家首次选育出“抗性淀粉”、蛋白质、膳食纤维含量高的水稻新品种﹣﹣“降糖稻1号”．“抗性淀粉”和普通淀粉相比，难以在消化道内降解成葡萄糖，不会使血糖过快升高．科学家利用糖尿病模型大鼠进行的相关动物试验证明，该稻米对于保持体重稳定增长、降低血糖有明显效果．

表是一些食物的pH，胃酸过多的病人空腹时最宜食用的是（　　）

物质	番茄	葡萄	牛奶	玉米粥
pH	4.0～4.4	3.5～4.5	6.3～6.6	6.8～8.0

质量为m的汽车，启动后沿平直路面行驶，如果发动机的功率恒为P，汽车行驶过程中受到的摩擦阻力大小一定，汽车速度能够达到的最大值为v，那么当汽车的车速为 $\frac{v}{4}$ 时，汽车的瞬时加速度的大小为（   ）

小球在斜面上向下滚下的过程中，小球的重力的示意图正确的是（　　）